Performance of gas nitrided and AlTiN coated AISI H13 hot work tool steel in aluminium extrusion

The performances of nitrided and AlTiN coated AISI H13 hot work tool steel samples were investigated under conditions similar to those encountered in aluminium extrusion. The compound layer on the surface of the gas nitrided tool steel is entirely removed during sliding contact with aluminium after a sliding length of 3900 m, under a contact load of 60 N exposing the underlying diffusion zone to the aluminium counterface. Aluminium is known to have a strong tendency to adhere to steel and forms an adhesive layer leading to a very high friction coefficient. This results in high shear stresses that are responsible for the fracturing of the diffusion zone which is intrinsically brittle owing to a dispersion of nitrides. Fracture events followed by spallation produces a substantial material loss in the nitrided sample. The AlTiN coating, on the other hand, resists delamination and protects the underlying steel substrate against a chemical interaction with the aluminium counterface. This is reflected by the wear rates measured to be 1.21 x 10(-3) mm(3) and 9.36 x 10(-7) MM3 for the nitrided and AlTiN coated samples, respectively. (c) 2012 Elsevier B.V. All rights reserved.